The exit pupil is the bundle of light rays emerging from the eyepiece into the observer's eye (Figure 5.1, p. 60). You can see it as a bright disk viewed from a few inches away (Figure 6.4). Its size depends on aperture and magnification:
Telescope aperture
Diameter of exit pupil =- -
Magnification
Telescope aperture
Diameter of exit pupil =- -
Magnification
For example, an 8-inch (20-cm) telescope at 100x has an exit pupil diameter of: 20 cm
Higher powers give smaller exit pupils, and any given exit pupil size corresponds to a fixed "power per inch" or "power per cm" of aperture:
Aperture 1
Exit pupil diameter Exit pupil diameter
For example, a 0.1-cm exit pupil corresponds to a magnification of 10 times the aperture in centimeters ("10x per cm").
Exit pupil size can also be found from f -ratio and eyepiece focal length:
Eyepiece focal length
Telescope f -ratio
Eyepiece focal length = Exit pupil diameter x Telescope f-ratio For example, a 32-mm eyepiece gives a 3.2-mm exit pupil with any f /10 telescope.
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